1. Field of the Invention
The present invention relates to a method of manufacturing a quartz resonator element in which on a quartz piece, a thin film-shaped electrode in order to excite the quartz piece is provided, a quartz resonator element, a quartz resonator, and a quartz oscillator.
2. Description of the Related Art
A quartz resonator element is an element constituted by a quartz piece and thin film-shaped electrodes each provided on both surfaces of the quartz piece, and is widely used for an electronic component of a quartz oscillator or the like as a reference source of a frequency or time by using a characteristic in which oscillation is excited by an inverse piezoelectric effect of quartz when a voltage is applied to the electrodes.
As external structures of a front surface side and a rear surface side are illustrated in FIG. 1(a) and FIG. 1(b), a rectangular quartz resonator element 1 using, for example, an AT cut quartz piece is constituted such that thin film-shaped electrodes are provided on both front and rear surfaces of a quartz piece 11 cut into a thin rectangular shape. Hereinafter, when an upper side is taken as a tip side and a lower side is taken as a base end side in FIG. 1(a) and FIG. 1(b), for example, rectangular excitation electrodes 12 and 13 are formed so as to face to each other via the quartz piece 11 in respective central regions close to tips on the both front and rear surfaces of the quartz piece 11. On the other hand, on base end portions of the quartz piece 11, for example, long rectangular lead electrodes 14 and 15 are provided separately from each other, and each of the lead electrodes 14 and 15 is formed on the both front and rear surfaces of the quartz piece 11 to stride over a side surface of the base end side of the quartz piece 11, for example. In this example, the excitation electrode 12 on the front surface side is connected to the lead electrode 14, and the excitation electrode 13 on the rear surface side is connected to the lead electrode 15.
FIG. 2 is a vertical cross-sectional view of a quartz resonator 2 in which the quartz resonator element 1 is stored in a package 21 being a surface mount-type receptacle. The package 21 is constituted as, for example, a flat casing provided with a rectangular plane shape that is one size larger than the quartz resonator element 1, and two electrodes 22 are provided separately from each other along, for example, one side of a rectangular bottom surface in the package 21. These electrodes 22 are connected electrically to an electrode 24 being a terminal portion provided on a bottom portion of an outside of the package 21. For example, the lead electrodes 14 and 15 formed on the base end portion of the rear surface side adhere to the two electrodes 22 respectively via, for example, a conductive adhesive 23, and thereby, the quarts resonator element 1 is fixed in the package 21 in a cantilever manner, and is connected to these electrodes 22 electrically. When this quartz resonator 2 is combined with an oscillation circuit and a voltage is supplied thereto, the quartz resonator element 1 is oscillated, and a frequency signal can be taken out from the electrode 24 being the terminal portion.
Here, in general, for the thin film-shaped electrodes constituting the excitation electrodes 12 and 13 and the lead electrodes 14 and 15, a metal such as gold or silver, which has small electrical resistance, is excellent in corrosion resistance, and has a relatively small oscillation loss (a CI (crystal impedance) value) when converting electric energy into oscillation energy, is used. However, the metal has weak adhesion to the quartz piece 11. Therefore, chromium having good adhesion to the quartz resonator element 1 is formed on the quartz resonator element 1 as a first metal layer, and then, thin films composed of gold or silver are formed on upper surfaces of the first metal layers as a second metal layer, resulting that a two layer structure is made. Thereby, adhesion of the electrodes 12 to 14 is improved (for example, Patent Documents 1 and 2).
Further, in the electrodes 12 to 14 having the above-described two layer structure, chromium has high adhesion to the conductive adhesive 23 compared with gold or silver, and therefore, a heat treatment in which the quartz resonator element 1 where the electrodes 12 to 14 are formed is heated to, for example, approximately 300° C. to diffuse the chromium of the first metal layer into an inside of the gold or the silver of the second metal layer is performed. The chromium is diffused into surfaces of the lead electrodes 14 and 15 by this treatment, and thereby, adhesion between the electrodes 12 to 14 and the conductive adhesive 23 can also be improved.
However, the present inventor has understood that chromium has large electrical resistance compared with gold or silver, and therefore, all of the electrical resistances of the electrodes 12 to 15 rise higher than those before the heat treatment is performed by the chromium being diffused into the metal. When the electrical resistances of the electrodes 12 to 15 become large, the CI value of the quartz resonator element 1 rises to thereby increase power consumption.
As for the above point, the second metal layer composed of gold or silver is relatively thickened to the first metal layer to suppress concentration of the chromium to be diffused into the second metal layer low, thereby enabling the rise in the electrical resistances of all of the electrodes 12 to 15 to be suppressed, but it is not advisable to thicken the metal layer of gold or silver because a manufacturing cost of the quartz resonator element 1 is increased.
Further, in recent years, a demand for the quartz resonator 2 having a high frequency increases, and the higher frequency the quartz resonator element 1 has, the thinner the quartz piece 11 needs to be. Therefore, in the case of the quartz resonator 2 having a high frequency of, for example, 50 MHz to 100 MHz, a thickness of the quartz piece 11 is thinned to, for example, approximately 33 μm to 16 μm. However, when the heat treatment is performed as is described already for the electrodes having the two layer structure provided on the quartz piece 11 that is thin as above, a frequency characteristic of the quartz resonator element 1 changes before and after the heat treatment due to a reason of stress exerted on the quartz piece 11 in the heat treatment or the like. Therefore, a desired oscillation frequency cannot be obtained, resulting that there is a problem that frequency adjustment is required again.
Note that arts described in Patent Documents 1 and 2 do not pay attention to a problem that the CI value is increased due to the diffusion of the chromium composing the first metal layer into the second metal layer, and further each electrode has a three layer structure to a four layer structure as a whole and a film of the whole electrode is thick, resulting that the problem that the frequency characteristic changes before and after the heat treatment due to thinning of the quartz piece 11 cannot be solved.